1. Field of the Invention
The present invention relates to a battery pack that houses a plurality of chargeable battery cells in a case and is used as the power source of portable equipment such as a video-camera, a portable personal computer, a digital camera, a mobile phone, PDA or the like.
2. Description of the Related Art
A conventional battery pack capable of charging is shown in FIG. 1. An example of the battery pack will be explained with reference to FIG. 1.
In FIG. 1, the battery pack is an approximately rectangular case made of insulative material such as plastic or the like, that houses a plurality of battery cells 13 composed of chargeable secondary battery cells such as, for example, a cylindrical nickel cadmium cell or nickel hydrogen battery, lithium ion battery, lithium metal battery or the like. Then, a positive electrode 13a and a negative electrode 13b of the battery cell 13 are respectively connected by resistance-welding to terminal boards 4-1, 4-2 made of a mono-metal plate of nickel or the like that has a favorable welding property. In addition, a protective circuit is incorporated therein for protecting the battery cell 13 and is led to outside terminals 15c, 15d of the positive and negative electrode in case a charging voltage becomes excessive.
Conventionally, the thickness of the metal plate of the positive and negative electrodes of the battery cell is about 0.10 mm to 0.2 mm or so. Additionally, as the material of the conductive terminal boards 4-1, 4-2 that are connected to the battery cells 13, nickel (Ni) is, for example, used that excels in terms of a resistance-welding property compared with copper (Cu) and aluminum (Al) that are conventionally used as wiring materials, and since fluctuations in welding strength occur when the thickness thereof is made thicker than 0.2 mm resulting in deterioration in connection reliability, a metal plate of about 0.15 mm or so in thickness is used.
Assuming that the thickness of each of the metal plates of the positive electrode and negative electrode of the battery cell is about 0.3 mm or so, connection reliability becomes higher even when the thickness of the material of the conductive terminal boards 4-1, 4-2 connected to the battery cells 13 is about 0.2 mm to 0.25 mm or so.
In this manner, the maximum value of the thickness of the material of the conductive terminal boards 4-1, 4-2 connected to the battery cells 13 depends on the thickness of the metal plates of the positive and negative electrodes of the battery cells 13.
Here, conventionally it is difficult to make the thickness of the metal plates of the positive and negative electrodes of the battery cells equal to or thicker than 0.25 mm when manufacturing costs, outer dimensions, and discharge capacity are taken into consideration.
For example, when the thickness of the metal plates of the positive and negative electrodes of the battery cell is made thicker, manufacturing costs become higher and discharge capacity deteriorates. This is attributed to the fact that there occurs a decrease in the cubic volume of electrode material that contributes to discharge capacity within the battery cell by the amount of an increase in the cubic volume of the metal plates of the positive and negative electrodes thereof.
As described above, since the conductive terminal plates 4-1, 4-2 are each connected by resistance-welding to the battery cells 13, that is, individual batteries in the battery pack, nickel is conventionally used as the material of the conductive terminal boards 4-1, 4-2. In addition, when this nickel is used, however, it is difficult to make the thickness thereof thicker than 0.15 mm in light of the above-mentioned connection reliability and further, since the thickness and width thereof are restricted due to limitations on housing space in a battery case, there is a limit for the conductive terminal boards 4-1, 4-2 consisting of the mono-metal of nickel to make wiring resistance smaller. Particularly, considering a growing demand of supplying a large amount of electricity of late, loss of energy due to electric resistance that the conductive terminal boards possess themselves becomes not negligible, which results in an inconvenience that can not be coped well with.
Therefore, as for the conductive terminal boards 4-1, 4-2 of the battery, propositions have been made as to what are described in patent documents 1 and 2. In the patent document 1 there is a description with respect to terminal wiring boards of a battery that are welded to the electrode terminal portions of individual batteries used in order to secure electric connections to the individual batteries constituting a set of batteries. The terminal wiring board includes a welding portion that has a mono-material structure of nickel and a conductive portion that has a multi-material laminated structure of nickel and copper or aluminum.
On the other hand, in the patent document 2, there is a description concerning wiring materials suitably used in a battery pack, a method of manufacturing thereof, and the like. The wiring materials are constituted such that conductive materials capable of metal welding such as iron (Fe), iron alloy, nickel (Ni), nickel alloy and the like are integrally joined in parallel with a plurality of heterogeneous highly conductive materials composed of conductive materials such as copper, copper alloy, and the like. As for the manufacturing method, a plurality of heterogeneous conductive materials composed of the above-mentioned materials are integrally joined such that after they are welded in parallel by means of electronic beam welding and then subjected to metal rolling in order to obtain a required thickness, annealing treatment is added thereto.
[Patent Document 1]
Japanese Laid-open Patent Application No. 2000-36299
(The second page, FIGS. 1a, b, and c)
[Patent Document 2]
Japanese Laid-open Patent Application No. 2000-229741
(The second page, FIG. 1)
However, with respect to the above-mentioned conductive terminal boards used in conventional battery packs, particularly regarding the one described in the Patent document 1, there occur such inconveniences that the thickness thereof becomes thicker as a whole because of a metal laminated structure and its resistance value becomes comparatively higher. In addition, in the one described in Patent document 2 there is also an inconvenience that the processes of manufacturing the conductive terminal board becomes complicated and difficult to be performed.